Sensing systems are employed for a wide variety of purposes and in diverse fields. There are sensing systems for detecting motion, surface irregularities, environmental conditions, and for physiological conditions, to name a few. Applications can be used in such industries as medical, process, aeronautical, and others. Such diversity in purpose and industry results in a multitude of considerations for the designer or user of the sensing system. These considerations include cost, precision, linearity, measurement range, durability, maintenance requirements, and even the physical characteristics of the sensed object, among others.
Non-contact, or non-invasive, sensing systems are sensing systems that, unlike direct contact sensing systems, do not require the sensing portion (e.g., sensor) to physically contact (directly or through an intermediary) the sensed, or targeted object. Non-contact sensing systems offer many advantages over traditional direct contact sensing systems, such as the ability to provide information regarding an object and/or condition of interest without expensive and invasive sensor mounting assemblies. Non-contact systems, unlike contact systems, also have the advantage of not changing the system they are measuring.
Radar systems using microwave energy are an example of one non-invasive sensing system. Radar systems use reflected electromagnetic waves, typically on the order of 0.9-100 gigahertz (GHz) to determine the presence, location, and speed of sensed objects. Continuous wave microwave techniques are non-contact, relatively inexpensive, and provide a sensing mechanism that is relatively unaffected by dust, debris, rain, and many other obscurants when the proper transmit frequencies are used. Another advantage of using microwaves is that the electromagnetic waves can be guided to the target to be measured; through mediums such as waveguide, circuit board, or coaxial cable. By having the microwave electronics at a distance away from the target, the electronics can be kept in a more environmentally controlled area, such as an enclosure, while the electromagnetic waves can be guided through the transmission medium to measure objects in less hospitable environments.
One disadvantage of using this microwave sensing technique is that the propagation medium used to guide the electromagnetic waves to the target contains metal, which can expand and contract in length over changes in temperature. When the coefficient of thermal expansion (CTE) is large, or a long length of transmission media is used, the change in length of the cable, board, or waveguide over temperature can yield significant errors in the displacement measurement. In high temperature environments, such as gas turbine engines, the temperature change of the antenna and transmission medium can change many thousands of degrees, yielding an unacceptable drift in the displacement output due to temperature.
In addition, this microwave sensing technique is only able to provide unambiguous measurements for distances of one-half wavelength or less to the target. If the distance from the transceiver to the target (including the length of the propagation medium) is greater than one-half wavelength, there is an ambiguity as to the number of integral wavelengths to the target. For example, if a target was 10.2 wavelengths from the transceiver, the output of the sensing system would indicate that the distance is 0.2 wavelengths to the target, since prior phase-based techniques only measure phase between 0-360 degrees (one-half wavelength of displacement).
Thus, a heretofore unaddressed need exists in the industry to address the aforementioned deficiencies and inadequacies.